The invention relates to apparatus and methods for loading wafers into cantilever diffusion tubes of the type described in commonly owned U.S. Pat. No. 4,459,104.
Commonly owned U.S. Pat. No. 4,459,104 discloses a cantilever diffusion tube apparatus including a quartz cantilever tube having a supported end clamped to a laterally movable carriage mechanism that in turn moves on a track. The cantilever tube has an outer or distal open end portion through which a plurality of spaced semiconductor wafers supported in a wafer boat are loaded into the cantilever tube. The cantilever tube is coaxially in alignment with a diffusion tube of a furnace. The proximal or support end of the cantilever tube is sealed by a door plate through which a gas conducting tube extends. The carriage then moves the cantilever tube and wafers supported therein into the diffusion tube. Reactant gases are caused to flow into the cantilever tube between the heated wafers and out of the cantilever tube. Purging gases are caused to flow through the cantilever tube and between the wafers therein while withdrawal of the cantilever tube from the diffusion tube occurs, avoiding excessive thermal shock to the wafers, premature exposure of the hot wafers to ambient oxygen molecules, and avoiding exposure of the wafers to ambient air containing defect-causing particles that can adhere to the wafers. The cantilever diffusion tube disclosed in U.S. Pat. No. 4,459,104 has a side window opening through which the wafer boat and wafers therein are loaded into and cut of the cantilever tube. To avoid problems associated with loading wafers through the side window, the slotted tube and loading apparatus and technique disclosed in commonly owned U.S. Pat. No. 4,543,059 was developed.
Although commercially quite successful, the slotted cantilever tube and associated wafer loading system disclosed in commonly owned U.S. Pat. No. 4,543,059 is more expensive than would be desirable. A plurality of relatively short wafer boats each abutting the other form a sealing cover for an elongated loading slot in the bottom distal portion of the slotted cantilever tube. The slotted cantilever tube itself is more expensive to manufacture than would be the case if the loading slot were omitted. The cantilever tube would be somewhat more rigid and less susceptible to damage if the elongated loading slot were not required. The slotted cantilever tubes are much more difficult to clean, as is frequently required, than would be the case if the elongated loading slots could be omitted, because the presence of the loading slots prevents cleansing liquids such as hydrofluoric acid from being conveniently "sloshed" around the inner surface of the slotted cantilever diffusion tubes in a safe manner. Three approaches have been developed for cleaning the slotted cantilever tubes. One approach is to provide a plastic or Teflon sheath that is wrapped around the slotted cantilever diffusion tube. The thus ensheathed slotted cantilever diffusion tube then is placed on a slighlty sloped tube roller machine that then causes the ensheathed tube to rotate. Hydrofluoric acid (HF) is sprayed into a slightly elevated end of the ensheathed cantilever diffusion tube as it is rotated by the tube roller machine, cleaning of the inside surface of the tube. Then de-ionized (DI) water is sprayed into the upper end of the rotating ensheathed cantilever diffusion tube to rinse out the hydrofluoric acid. Although tube rollers are commonly used to to clean quartz diffusion tubes and the like, they obviously cannot be used in cantilever diffusion tube with elongated loading slots without ensheathing them in the above-mentioned Teflon or plastic sheath. Use of such ensheathing is quite inconvenient, and constitutes a deterrent to adoption of the cantilever diffusion tube system by perspective customers. Another approach to cleaning slotted cantilever diffusion tubes is to support them by hand, selectively spraying HF to cleanse the inside surfaces and then rinsing them with sprayed DI water. This approach is very labor intensive, and presents substantial safety hazards to the workers. The last approach is to support the slotted cantilever diffusion tube vertically and selectively spray HF and then rinse with DI water; again, this approach is very labor intensive, is rather unsafe, and is very unsatisfactory.
Carefully controlled experiments have been performed using the slotted cantilever diffusion tubes of commonly owned U.S. Pat. No. 4,543,059, and have shown that some leakage of gas into the cantilever diffusion tubes occurs through the elongated loading slot, despite the careful design and positioning of the wafer boats to seal the elongated loading slot. The experiments show that shifts of surface state charge (Q.sub.SS) in test wafers are greater in portions of wafers that are nearest to the gaps associated with the elongated wafer loading slots of the slotted canti1ever diffusion tubes. Those skilled in the art know that such Q.sub.SS shifts are a strong measure of the density of oxygen molecules to which the wafers are exposed when still hot, usually during the process of unloading wafers from a diffusion furnace. Q.sub.SS shifts are a cause of substantially reduced integrated circuit manufacturing yields.
Another deterrent that stands in the way of adoption of the cantilever diffusion tube system is that the specialized wafer boats, with semicylindrical bottom surfaces designed to seal the wafer loading slot, cannot be used with any of the half dozen or so popular wafer transfer machines that transfer wafers from quartz diffusion boats into plastic wafer carriers that are used in various "wet chemistry" processing operations in integrated circuit manufacturing processes.
For some semiconductor processing operations, it would be ideal to have a system wherein boat loads of wafers are entirely encapsulated and sealed from outside gases during loading into a diffusion furnace, during the heat treatment within the furnace, and during unloading of wafers from the hot zone of the furnace and subsequent cooling. Applicant's prior cantilever diffusion tube systems go a long way toward accomplishing this goal, but do not entirely eliminate effects of outside gases, especially oxygen molecules, during withdrawal of the cantilever diffusion tubes from the hot zones of diffusion furnaces. The only system that provides a completely sealed environment for semiconductor wafers during loading, heat treatment, and unloading processes is a system proposed by IBM many years ago, in which the wafers, along with a source that releases reactant gases at high temperatures inside a diffusion furnace is entirely encapsulated in a quartz ampule. After the heat treatment has been completed, the quartz ampule is struck with a hammer and shattered to allow removal of the wafers. This technique generates quartz dust, which is a source of defect-causing particles that subsequently reduce the manufacturing yield. Furthermore, this technique is expensive because the quartz ampules cannot be re-used.
There is an unmet need for a more practical technique for enclosing wafers in a controlled ambient during loading into a diffusion furnace, heat treatment therein, and unloading.
The previously disclosed wafer loading systems generally require multiple wafer loading operations to load several relatively short boat loads of wafers into the cantilever diffusion tube. As indicated above, some leakage of reactant gases through the seal formed by the round bottoms of the specialized wafer boats that cover the elongated slots inevitably occurs, increasing the likelihood of introducing defects into the wafers being processed and, in some cases, causing more rapid contamination of the main furnace diffusion tube than would otherwise be the case. The previously described cantilever diffusion tubes have the problem that systems for automating the loading of wafers therein are considerably more complex and expensive than would be desired.
Despite the great advance in the art represented by the previously described cantilever diffusion systems, there remains an unmet need for further refinement of the overall system to further reduce the effects of outside gases on the wafers, to further reduce contamination of the main furnance diffusion tube, and to make use of the cantilever diffusion tube approach less expensive and more susceptible to automation especially with respect to wafer loading techniques.